Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Chemical Physics, 2007, 126 (1), 014104 and may be found at: http://dx.doi.org/10.1063/1.2405355

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One of the authors (A.J.A.) is grateful for the support of EPSRC under Grant No. GR/S28631/01. Two of the authors (D.P and L.R.) acknowledge support from the Marie Curie program of the European Union, Contract No. MRTN-CT2003-504712.